It has been found that amplitude and delay distortion resulting from multipath fading can, under certain conditions, be a major cause of transmission deterioration in both digital and FM radio communications systems. Experimental and analytical evidence indicate the outages of wideband digital systems, caused by this phenomenon (i.e., selective fading), may exceed system performance objectives. Due to the frequency selectivity of the multipath fades, transmission error rates can exceed design objectives much more frequently than is predicted from consideration of fades solely resulting from a drop in total received signal power. This means that techniques for handling flat fades, such as AGC, will not be adequate to maintain satisfactory wideband digital transmission, and that additional correction will be required. Even the use of space diversity techniques and/or frequency-responsive equalization will not eliminate all of the problems associated with selective fading.
The problem, however, is that signal fading resulting from multipath transmission is basically unpredictable not only for when it occurs but also in the magnitude of the transmission deterioration experienced. Accordingly, compensation for carrier and timing recovery techniques must be capable of measuring transmission parameters independently of carrier signal phase. The measured parameters may then be used to implement a matched filter responsive to changing signal conditions. If such compensation techniques utilize analog correlation, it advantageously would not be restricted to a particular modulation format.